The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Compression-ignition engines, such as diesel engines, operate by directly injecting a fuel into a combustion chamber. Manufacturers of compression-ignition engines have developed combustion modes for reducing engine-out emissions. Exemplary combustion modes include premixed charge compression ignition (‘PCCI’) and homogenous charge compression ignition (‘HCCI’). The combustion modes can utilize advanced injection timing along with high rates of exhaust gas recirculation to create a premixed or more homogenous combustion mixture at the time of combustion. Compression-ignition engines utilizing the advanced combustion modes may exhibit increased pressure levels and increased pressure level variations within combustion chambers over other known compression-ignition engines. Compression-ignition engines utilizing the advanced combustion modes may exhibit high noise levels. High noise levels can be caused, for example, by oxygen, fuel, and pressure conditions inside the combustion chamber that are outside a desired range causing insufficient or mistimed combustion, and mistimed fuel injection within the combustion chamber. Pressure levels can vary among combustion cycles in engines utilizing high levels of recirculated EGR. Engines utilizing advance combustion timing can have higher pressure rise rates, which can result in increased noise levels. Known compression-ignition engines impose calibrated limits on air and fuel quantities over an operating range to control noise levels.